The present invention is generally related to signal processing. More particularly, the present invention is related to methods and systems providing waveform synthesis for imaging and ranging applications such as RADAR, SONAR, LIDAR, medical imaging, tomography, and communications applications utilizing spread spectrum modulation/demodulation techniques.
Although the present background describes the functionality and limitations of synthetic aperture radar systems or a particular class of communications, such description is merely provided to exemplify a problem capable of resolution with the present invention. Any discussion herein directed to specific radars or communications protocols should not be taken by those skilled in the art as a limitation on the applicability of the invention described herein.
Modern high-performance radar systems often generate signals of extraordinarily wide bandwidth. For example, the General Atomics Lynx Synthetic Aperture Radar (SAR) employs a Linear-FM (LFM) chirp waveform and can operate over 3 GHz bandwidth at a 16.7 GHz center frequency. Furthermore, maximum exploitation of these radar signals requires the generated waveforms to be of very high quality, possessing exceptional spectral purity.
To facilitate high-quality LFM chirp generation, a programmable Digital Waveform Synthesizer (DWS) can often be employed. Use of a GaAs ASIC has been shown to implement well known double accumulator architectures to generate a phase that is quadratic with time, a phase that is generally converted by a memory look-up table to a digital sinusoidal signal and is ultimately converted to an analog signal by a Digital to Analog Converter (DAC). Furthermore, an ability to predistort the phase of the output as a function of instantaneous frequency to correct for unspecified nonlinearities of subsequent components in the signal path has previously been explored. No calibration scheme, however, has been presented for determining correction factors.
Two principal architectures are presently employed for achieving LFM chirp generation. The first architecture 100 is referred to as single-ended output operation, and is illustrated in FIG. 1 (labeled as prior art). With this architecture a single signal output is generated by the DWS 110 and presented to subsequent components in the signal path. After mixing 115 with a Local Oscillator (LO) 120 signal the nature of a single-ended DWS signal is to generate the desired signal as well as an undesired mirror-image signal, which must be filtered by analog components in the signal path. This filter is often called a sideband filter 130. Consequently, desired and undesired signals are separated by frequency; limiting the usable bandwidth for a generated signal to something less than half the DWS clock frequency. Equivalently, a clock frequency of more than twice the highest output waveform frequency is required. Proper final system bandwidth is achieved through a frequency multiplier 140.
Multiplexing of multiple parallel chirp generators can allow wide bandwidth single-ended chirps to be generated with commercial silicon Field Programmable Gate Array (FPGA) components. Frequency multiplication can also be employed to widen the bandwidth of a single-ended DWS output signal, but often to the detriment of spectral purity. It is also well known that frequency multiplication raises undesired frequency spurs by 6 dB per doubling with respect to the desired signal level. Frequency spurs, however, are undesired signal perturbations caused by quantization effects and DAC residual nonlinearities. Consequently, minimizing the frequency multiplication factor that can be applied to a DWS output can enhance spectral purity.
A second architecture that has been used for achieving LFM chirp generation quality can be referred to as balanced or quadrature modulator operation. Such architecture is generally illustrated in FIG. 2 (also labeled as prior art). With this architecture 200, two output signals are generated by the Quadrature DWS (QDWS) 210 and presented to a Single Sideband (SSB) mixer 220 where they are combined 215 to form a single signal to the subsequent signal path. In a perfect system, the two signals generated by the QDWS 210 will differ by a constant 90 degrees of phase, and are termed the In-phase (I) and Quadrature-phase (Q) signals. The signal pair together can be generally referred to in the art as Quadrature signals. In a perfect SSB mixer 220, no mirror-image signal will be generated, obviating the need for a sideband filter. Furthermore, no spectral separation between desired and a nonexistent undesired signal would need to be maintained. Consequently, the QDWS 210 output bandwidth of the desired signal would be able to approach the QDWS clock frequency itself, which is twice the bandwidth of the single-ended DWS system. This, in turn, would require half the frequency multiplication 230 when compared to a single-ended DWS system to achieve a final system bandwidth, and include attendant 6 dB lower spur levels and better spectral purity.
SSB signal generation techniques, including the employment of quadrature signals, are generally known in the art. Quadrature signals can be generated by a variety of techniques, including Hilbert filters that generate a 90-degree phase shift for all input waveform frequencies, and directly by separate memory look-up tables within the digital signal generation portion of the QDWS. The precision with which quadrature signals can be generated and combined in a SSB mixer, however, is problematic, particularly for high-dynamic-range applications such as imaging radar systems. Imperfections in quadrature signal generation or their combination within a SSB mixer results in the non-cancellation of the undesired mirror-image sideband signal. Such imperfections can result in a relative phase error or an amplitude imbalance. Additionally, the LO 120 may undesirably leak through the mixer and be present in the mixer output in addition to the desired signal. Any of these errors reduce the spectral purity of the resulting SSB mixer output signal, and degrades a SAR image with ghosts and other artifacts. Consequently, quadrature modulators in high-performance radar systems require some form of error cancellation or other mitigation scheme.
In the field of communications, a quadrature modulator for wireless CDMA systems has been described wherein amplitude and phase of the quadrature component signals are predistorted to provide perfect quadrature signals to the SSB mixer. Furthermore, DC biases are added to the quadrature output signals to mitigate LO leakage. The corrections, however, are derived for a single QDWS output frequency and do not allow for frequency dependent errors. While this may be reasonable for applications such as wireless communications, it is inadequate for high-performance SAR systems. Furthermore, no attempt is made to compensate imbalances in the SSB mixer itself, and the stated procedure precludes this.
An iterative procedure to adjust for LO leakage, and phase and gain imbalance in a quadrature modulator based on an envelope detection-of a transmitted signal has also been described by the prior art. Other prior art techniques include: use of a similar technique to compensate an SSB mixer-which degrades a resultant output signal and adaptive techniques for achieving quadrature signal balance using a test tone. The adaptation procedures currently described in the prior art is not adequate for wideband LFM chirps. Furthermore, verified wideband frequency-dependent errors are not addressed by the prior art.
A quadrature modulator that allows frequency-dependent phase and amplitude corrections to be made to the output of the QDWS has been proposed. All corrections are made to analog signals after the DACs. These corrections, however, neglect problematic frequency dependent errors in the SSB mixer. Furthermore, the nature of the errors to be corrected is presumed to be predeterminedxe2x80x94that is, no calibration procedure is discussed.
A quadrature modulator has been described that can be constructed that implements frequency dependent corrections before and at the DACs. The SSB mixer, however, is not addressed, nor is correction due to imbalance from any components or sources addressed.
Thus far, quadrature modulator prior art generally is inadequate for high-performance radar systems for one or more of the following reasons:
1) The balance corrections are relatively narrowband, and do not facilitate frequency dependent phase and amplitude corrections.
2) Adaptive schemes are not fast enough for LFM wideband chirp waveforms:
3) Imbalances within the SSB mixer itself are not addressed.
4) LO leakage (e.g., especially any frequency dependence) is not adequately addressed.
5) Where calibration schemes should be required, no calibration schemes are proposed.
The prior art leaves the skilled in the art with limited choices between narrow-band quadrature modulators that can compensate for imbalances and LO leakage using iterative techniques and wideband quadrature modulators that do not compensate for frequency dependent errors in the SSB mixer. Accordingly, there is a need for an improved wideband quadrature modulator and SSB mixer combination, which produces suitable signals for use in high fidelity radar waveform generation. Additionally, there is also a need for an operational methodology that is insensitive to the imbalances and leakage in a quadrature modulator and associated SSB mixer.
In the field of quadrature demodulators, which are used in radar receivers, U.S. Pat. No. 6,469,661, issued Oct. 22, 2002 to two of the present inventors teaches how errors due to I and Q channel imbalances in a receiver""s demodulator can be mitigated by processing radar signals with pulse-to-pulse, rolling phase shifts. With this technique no calibration is required to achieve a balanced demodulator output signal. The use of quadrature modulators in radar transmitters, however, has not been described. The present inventors have discovered how use of a Quadrature DWS is viable for high-performance radar waveform synthesis. A principal advantage of the present invention is that fewer frequency multipliers are needed to generate radar waveforms of sufficient bandwidth; thereby offering the potential of cleaner waveforms (e.g., signals processing lower frequency-spurs).
Problems that generally need to be resolved with QDWS signals are those related to the generation of unwanted signal components due to energies such as LO feed-through found in the non-ideal SSB mixer and quadrature signal imbalance, as well as problem typically caused within the SSB mixer. For example, the inclusion of unwanted energy into a radar""s output (e.g. SAR images) creates problems associated with the QDWS and SSB components that result in signal waveform degradation, and hence radar performance. Similar problems are encountered with other systems engaged in or requiring signal/waveform synthesis, such as: SONAR, LIDAR, medical imaging, tomography, and communications.
It is a feature of the present invention to provide an improved Quadrature Digital Waveformn Synthesizer that can further provide systems with frequency dependent corrections for quadrature imbalance and Local Oscillator (LO) feed-through.
It is another feature of the present invention to provide operational procedures to system that can enable filtration of quadrature imbalance effects from energies, such as LO feed-through energy and imbalance energy, and their effects without prior calibration or equalization.
It is yet another feature of the present invention to provide calibration procedures that can also be implemented into systems for signal synthesis.
Techniques that are herein disclosed by the present inventors to mitigate the negative effects of signal components brought on by unwanted energies such as LO feed-through and quadrature signal imbalance include the benefits of:
1) Compensating or equalizing energy imbalance by adjusting inputs (DC offsets, phase, and amplitude) to the SSB mixer so that problematic signal components aren""t generated, and
2) Operating in a manner (e.g. by employing rolling phase shifts) that separates problem signals (i.e., energy) from a desired signal in Doppler to allow filtering to suppress problem energy.
In accordance with methods of the present invention, waveform generation in a system can be adjusted/corrected, for example, in a synthetic aperture radar system (SAR), where a rolling phase shift is applied to the SAR""s QDWS signal where it is demodulated in a receiver; the LO feed-through energy and/or imbalance energy are then separated from a desired signal in Doppler, the separated energy is filtered from the receiver leaving the desired signal; and the separated energy in the receiver is measured to determine the degree of imbalance that is represented by it. The degree of imbalance in the system can be used to determine calibration values that can then be provided as compensation for frequency dependent errors in components, such as a SAR""s QDWS and SSB mixer, affecting quadrature signal quality.
Error correction circuits can be built into the QDWS that compensate wideband frequency dependent phase and amplitude imbalances originating in the QDWS, SSB mixer, and other non-ideal components in the I and Q channels of the quadrature modulator. This can be done by predistorting the relative phase and amplitudes of the QDWS quadrature signal components by programmable frequency dependent digital perturbations into the QDWS prior to analog signal generation. LO leakage in the SSB mixer is thereafter compensated by applying DC offsets to the SSB inputs.
Frequency dependent phase and amplitude corrections and LO leakage compensation can be determined by a calibration process that involves applying pulse-to-pulse rolling phase shifts to the LFM signal within the QDWS. These phase shifts can be removed in the receiver by conjugate phase shifts. The application of these phase shifts can separate desired signals from differential-mode imbalance energy in a Doppler spectrum (the spectrum taken across the pulses). Separation allows for the unique identification, of the nature of the quadrature imbalance and LO leakage as a function of frequency for calibration purposes.
Alternatively, the present inventors teach that the calibration process can be dispensed with, where Doppler filtering can be applied to the received data to simply remove the imbalance, such as LO leakage energy, from the received signal. Calibration can thereby be rendered unnecessary, rendering any quadrature signal imbalances, or LO leakage, impotent.